The Rose Manufacturing Safe-Hi is an old design that is so different from anything else in my collection that it took me a long time to figure out how it was rigged. The device can be opened by releasing the rope catch (blue arrow) and pulling a lot of slack into it. The two main pieces can then be separated, using the two "curls" (green arrow) as a hinge, and then sliding the two pieces sideways to separate them completely. The main line is placed in the notch, and the two main pieces are reassembled. The rope is now running along the path indicated by the red arrow. The lanyard is pulled tight and held their by the rope catch.
When the worker takes the attempted suicide plunge, the lanyard tightens and pulls the two main pieces together. This squeezes the main line, bringing the worker to a stop and averting the big splat. The instructions on the device indicate that 15 feet (5 meters) of clear space are needed for this to happen.
I don't see how this principle could be used in a working ascender, but perhaps a rappel safety could utilize the idea.
The Rose Manufacturing Safe-Hi is protected by U.S. Patent 2,914,139.
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©2007, Gary D. Storrick